This invention generally relates to apparatuses, systems and methods used in crude oil desalting processes and, more specifically, to systems and methods used to inject dilution water into a crude oil stream in order to contact and coalesce entrained water within the stream.
The crude oil desalting process involves washing a crude oil stream with water having a low salt content (e.g. typically about 250 ppm or less) followed by electrostatic dehydration of the resulting mixture. The washing step involves mixing the low salt-content (“fresh” or “dilution”) water with the crude oil stream so as to add energy into the stream and coalesce the dilution water with the brine water already entrained in the crude oil stream.
Mixing is accomplished through a mixing valve, static mixer, or some combination of the two. The degree of emulsification of the dilution water primarily depends on the pressure drop imparted by the valve. A normal design range for this pressure drop is in a range of 5 to 25 psi, with most valves or mixers operating below 15 psi. If too large of a pressure drop is created, the water droplets decrease to a size which makes them difficult to coalesce and remove in the downstream electrostatic dehydration process. A pressure drop control system, like that shown in FIG. 1, is used to control and operate the drop within the critical range.
Prior to the crude oil stream entering the mixing valve, it is advantageous to disperse the dilution water in the oil phase. This is typically done by way of a disperser which uses medium pressure spraying of the dilution water through holes on the dispersing tube of the disperser at a rate of 3-10% of oil flow rate. The spraying occurs in a direction perpendicular to the flow of the crude oil stream (see FIGS. 2A & B). Use of a static in-line mixer has also proved beneficial in accomplishing this dispersion.
One problem with the prior art dispersion system and method is, the dilution water droplets being sprayed or dispersed into the crude oil stream are greater than 1000 microns in size. In the invention described below, spray nozzles atomize wash water into the crude oil stream. The atomized water droplets are in a size range of 10 to 300 microns. This smaller wash water droplet size works to increase the contact efficiency with the brine droplets contained in the crude oil stream, thereby increasing desalting performance.